Many systems use batteries each formed of a battery pack or battery array, including a plurality of battery cells connected in series with each other.
When such battery cells are charged to voltages significantly higher than voltages within a rated charge range or discharged to voltages lower than voltages within a rated discharge range, they may be dangerous.
The imbalance between the charged states of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge/discharge of batteries. In the case of lithium ion cells, the manufacture of cells is strictly controlled in a factory to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality was achieved in a factory after the cells were initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, the reduction of the capacities of the cells, variation in the operating temperatures of the cells, and different types of variation between the cells.
The inconsistency between the temperatures of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature over time.
Imbalance is a very series problem in the charged state of a battery. For example, the ability of a battery to supply energy is limited by a battery cell having the lowest charged state, which may typically occur in electric vehicles.
If the battery cell is fully consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells of the battery still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply ability of the battery.
Of course, the above description does not mean that, when one or more battery cells are consumed, the supply of power by the remaining battery cells is completely impossible. However, it means that, in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell for which discharge has been completed, with the result that the battery cell may be in danger of explosion due to the overheating thereof or the generation of gas, and thus the battery loses power supply ability.
Various methods of correcting the imbalance between the charged states of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing a conventional charge equalization apparatus.
Referring to FIG. 1, the conventional charge equalization apparatus includes transformers T1 to Tn, switches SW1 to SWn, diodes D1 to Dn, and a voltage detection and drive signal generation unit 10. Then, dots on a primary winding and on a secondary winding in the transformers are located at opposite sides. Accordingly, each transformer is operated such that, when the primary winding is turned on, energy is charged in the primary winding, whereas, when the secondary winding is turned off and the primary winding is turned off, the energy charged in the primary winding is supplied to the secondary winding by a counter electromotive force, and thus the secondary winding is turned on.
The switches SW1 to SWn are connected in series with the primary windings of the transformers T1 to Tn, respectively, and are adapted to supply energy charged in overcharged batteries to the primary windings of the transformers T1 to Tn in response to a control signal provided by the voltage detection and drive signal generation unit 10. Accordingly, the overcharged batteries are discharged.
When the switches SW1 to SWn, connected in series with the primary windings, are turned on, the transformers T1 to Tn are supplied with energy by overcharged batteries, and charge the energy in the primary windings thereof. When the switches SW1 to SWn are turned off, the transformers supply energy, charged in the primary windings, to the secondary windings thereof by counter electromotive force.
The diodes D1 to Dn are connected in series with the secondary windings, respectively, and are adapted to rectify energy supplied from the secondary windings to the series-connected batteries B1 to Bn.
The voltage detection and drive signal generation unit 10 detects the voltages of respective series-connected batteries B1 to Bn, compares the detected voltages with a reference voltage, and generates a drive signal required to discharge energy from batteries, which are charged to voltages greater than the reference voltage, that is, overcharged batteries. The drive signal generated by the voltage detection and drive signal generation unit 10 is provided to switches connected in parallel with the overcharged batteries, thus forming a closed loop to allow the energy of the overcharged batteries to be supplied to the primary windings of the transformers T1 to Tn.
However, the conventional charge equalization apparatus is problematic in that, since the diodes D1 to Dn are connected in series with the secondary windings, voltages which are about twice the voltages of the series-connected batteries B1 to Bn are induced at the diodes D1 to Dn when the switches SW1 to SWn are turned on, thus increasing voltage stress on the diodes D1 to Dn. In other words, when the switches SW1 to SWn are turned on, both voltages supplied from overcharged batteries to the primary windings of the transformers and voltages supplied from the series-connected batteries B1 to Bn are induced at the diodes D1 to Dn, so that high voltage stress is applied to the diodes D1 to Dn. Accordingly, the withstand voltages of the diodes D1 to Dn connected in series with the secondary windings of the transformers T1 to Tn increase, and thus the cost of the charge equalization apparatus increases.
In addition, in the conventional charge equalization apparatus, since the number of turns in the secondary windings of the transformers T1 to Tn is N times (N is the number of series-connected batteries) the number of turns in the primary windings, there are many limitations in the implementation of secondary windings as the number of batteries increases.